As stated in the Overview video, we're starting from the top half of the scale and only playing that half. The reasons for doing this are to gradually tackle the scale, which for many is your first foray into playing a scale (with alternate picking no less), to be able to start on a downstroke (which for many is most comfortable in these beginning stages) and to get you in the right head when you start to improvise your own licks and solos. In regards to the latter, many players first improvise from first impressions and, much more often than not, scales are always presented from the bottom and played in an ascending fashion. While that's all well and good the action for guitarists when soloing is mostly in the upper half on the unwound strings.